1. Field Of The Invention
This invention relates to a method and apparatus for making a superconducting joint and particularly to such a method and apparatus for making a superconducting joint between multifilamentary superconducting wires with similar and/or different matrix materials.
2. Discussion Of The Prior Art
It is well known that the resistance to current flow of many materials falls to zero, thereby making them superconducting, when they are cooled to a low temperature, known as the critical temperature. Superconducting wires typically comprise a superconducting material embedded in a matrix material, which may be, for example, copper or cupro-nickel, which is not itself a superconducting material. In the case of wire made of niobium-titanium alloy, which is a common superconducting material, the alloy may be in the form of filaments and the filaments encased by the matrix material.
It is necessary to form joints which are themselves superconducting to be able to usefully incorporate such superconducting wires into a superconducting circuit. For example, the wire may be wound into a coil to produce a high magnetic field, such as in an MR scanner. The coil cannot be made in a loop because it is necessary to be able to turn such a coil on and off. For that purpose, a superconducting switch is connected between the ends of the coil. To make the connection, it is necessary to make superconducting joints between the switch leads and the coil leads which result in a continuous circuit which is superconducting at the critical temperature and in a high magnetic field.
Methods are known for joining superconductors. One such method in disclosed in Thornton U.S. Pat. No. 4,584,547, which is owned by the assignee of the present invention. Thornton describes a method of making a superconducting joint which uses one or two pots of molten metal to etch the matrix material away from the filaments and then solder the filaments of the superconductors to be joined together. It is also known to use such a process to mold the filaments of the superconductors to be joined together with the molten metal while rotating each solder pot. This method is fully disclosed in Kumpitsch et al. U.S. Pat. No. 4,713,878, which is also owned by the assignee of the present invention. Typically in these processes, the etching metal used was tin and the soldering/molding material was lead-bismuth.
However, such processes were best performed in a controlled environment, such as a glove box, to insure producing good quality joints. Otherwise, oxides and other contaminants were believed to interfere with the joint and make it non-superconductive.
In addition, it has been difficult to join superconductors of one matrix material to superconductors of another matrix material, even if the superconductor material was the same. For example, copper matrix niobium-titanium multifilamentary wire was difficult to join to cupro-nickel matrix multifilamentary wire.